The present invention relates to an electrophotosensitive material comprising an intermediate layer (undercoat layer) formed between a conductive substrate and a photosensitive layer.
As an electrophotosensitive material for use in image forming apparatuses such as electrostatic copiers, plain paper facsimiles, laser beam printers and combined devices having these functions, a so-called organic electrophotosensitive material is widespread which comprises a combination of the following components:
a charge generating material for generating electric charges (positive hole and electron) when exposed to light;
a charge transport material for transporting the generated electric charges; and
a binder resin.
The charge transport materials fall into two broad categories which include a hole transport material for transporting positive holes of the electric charges, and an electron transport material for transporting electrons.
The organic electrophotosensitive material has an advantage over an inorganic electrophotosensitive material employing an inorganic semiconductor material in that the organic electrophotosensitive material is fabricated more easily at less production costs than the latter.
In addition, the organic electrophotosensitive material also has a merit of greater freedom of function design by virtue of a wide variety of options for materials including those described above.
In this connection, the organic electrophotosensitive materials have recently been widely used in the image forming apparatuses.
The organic electrophotosensitive material is fabricated by forming either of the following photosensitive layers on a conductive substrate:
A single-layer photosensitive layer containing a charge generating material, charge transport material (hole transport material and/or electron transport material) and binder resin; and
A multi-layer photosensitive layer in which a charge generating layer containing a charge generating material, and a charge transport layer containing a charge transport material (hole transport material and/or electron transport material) are laminated in this order or vise versa.
Unfortunately, these photosensitive layers encounter the following problems when formed directly on the conductive substrate.
(a) In a charging step of the image formation, when a surface of the photosensitive layer is positively or negatively charged, a charge of the opposite polarity to the above occurs in the conductive substrate.
The photosensitive layer formed directly on the conductive substrate, however, is susceptible to the injection of the electric charge of the opposite polarity from the conductive substrate. If a large quantity of electric charge of the opposite polarity are injected into the photosensitive layer, the total amount of electric charge at the photosensitive layer surface is lowered.
Accordingly, an electrostatic latent image formed on the photosensitive layer surface in the light exposure step has a decreased potential difference between a light exposure region and a non-exposure region. This causes a printed image to sustain fogging due to the adhesion of toner particles to white areas thereof.
(b) The single-layer photosensitive layer or the lower layer of the multi-layer photosensitive layer is formed by applying a coating solution containing the above components onto the conductive substrate, followed by drying the coating film. In some cases, however, the resultant layer may be insufficiently bound onto the conductive substrate depending upon the type of the binder resin or the solution application conditions. This results in the delamination of the formed layer.
(c) If a surface of the conductive substrate contains a defect such as mars, the surface of the photosensitive layer formed directly on the conductive substrate will also sustain a similar defect. This defect causes black spots or white spots in the formed image. Whether the defect results in the black spots or the white spots depends upon whether the image forming process adopts the normal development method or the reversal development method.
With an aim at solving these problems, there has been proposed an electrophotosensitive material wherein an intermediate layer containing a binder resin is formed on a conductive substrate, and then a photosensitive layer is laid thereover.
By virtue of the intermediate layer so provided, this electrophotosensitive material is adapted to prevent the electric charge of the conductive substrate from being injected into the photosensitive layer, to achieve a firm bond between the conductive substrate and the photosensitive layer, and to cover up the defect in the surface of the conductive substrate for a smooth, defect-free surface of the photosensitive layer.
A thermosetting resin is preferably used as the binder resin for forming an intermediate layer having good thermal, chemical, physical and mechanical stabilities, especially insolubility in the dispersion medium (ex. organic solvent) contained in the coating solution for the photosensitive layer, and an excellent binding with the conductive substrate.
The intermediate layer containing the thermosetting resin is formed by applying a coating solution, such as prepared by dissolving or dispersing the thermosetting resin in a dispersion medium, to the surface of the conductive substrate and then heat treating the coating film for curing the thermosetting resin.
However, if the heat treatment is insufficient, the curing degree of the thermosetting resin is lowered so that the intermediate layer is decreased in the thermal, chemical, physical and mechanical stabilities. When a coating solution for photosensitive layer is applied to such an intermediate layer in order to overlay the photosensitive layer thereon, the intermediate layer will be dissolved or changed in properties by a dispersion medium contained in the coating solution. This may result in nonuniform thicknesses or inhomogeneous compositions of the photosensitive layer.
With a low curing degree of the thermosetting resin, the intermediate layer tends to be decreased in conductivity, leading to the likelihood of fog.
In the light exposure step of the image formation, the charge generating material generates both the positive and negative charges in the light exposure region of the photosensitive layer. One of the charges is transported to the conductive substrate while the other charge negates a charge potential of the surface of the photosensitive layer so that an electrostatic latent image is formed on the surface of the electrophotosensitive material in correspondence to a light exposure pattern.
However, if the intermediate layer between the photosensitive layer and the conductive substrate has a low conductivity, the charge (of the same polarity as that of the surface of the electrophotosensitive material) to be transported to the conductive substrate is blocked by the intermediate layer, thus remaining in the photosensitive layer.
Therefore, the electrophotosensitive material is increased in residual potential so that the printed image tends to sustain fog in its white area.
Furthermore, the intermediate layer is associated with another cause of fog. That is, a de-electrification step subsequent to an image transfer step cannot sufficiently eliminate the charge of the surface of the photosensitive layer because of the interference of the intermediate layer so that the photosensitive layer is increased in residual potential.
The present inventors have studied an approach wherein prior to the formation of the photosensitive layer, the curing degree of the thermosetting resin forming the intermediate layer is non-destructively determined so as to preclude any intermediate layer containing the thermosetting resin insufficiently cured. Then, the photosensitive layer may be laid only over an intermediate layer containing the thermosetting resin sufficiently cured.
Japanese Laid-open Patent Publication No. JP05-19518A (1993) discloses a method which comprises the steps of: applying a coating solution for surface layer on a photosensitive layer, the coating solution containing a polyester resin, an epoxy resin reactive with the polyester resin, and a photo polymerization initiator; irradiating the coating film with an actinic radiation for initiating the polymerization; and quantitatively determining the curing degree of the epoxy resin when the polymerization reaction is completed by heat aging thereby to form the surface layer.
In an infrared absorption spectrum, an absorption peak of a carbonyl group originating in the polyester resin is in a nearly saturated state. Therefore, a residual epoxy group in the epoxy resin is determined for relative absorption intensity at absorption peak based on the absorption peak of the carbonyl group, thereby to quantitize the curing degree of the epoxy resin.
However, the application of the above method is limited to the above composition, which is suitable to form the surface layer but not suitable to form the intermediate layer which is required of the aforesaid various characteristics. Hence, there has been a need for developing a novel method of determining the curing degree of the thermosetting resin contained in the intermediate layer.
It is an object of the invention to provide an electrophotosensitive material adapted to achieve a low residual potential of a photosensitive layer for offering fog-free favorable images because an intermediate layer contains a sufficiently cured thermosetting resin as a constituent thereby achieving good thermal, chemical, physical and mechanical stabilities, and also has a suitable conductivity.
Another object of the invention is to provide a method for producing the above electrophotosensitive material having favorable characteristics in high yield.
Still another object of the invention is to provide an inspection method in which measurement is taken on an intermediary before forming the photosensitive layer on the intermediate layer of an electrophotosensitive material for determining the curing degree of a thermosetting resin forming the intermediate layer, thereby classifying the intermediary into an acceptable one having a high curing degree and a defective one having a low curing degree.
In the pursuit of the above objects, the present inventors have examined a factor having a correlation with the curing degree of the thermosetting resin and permitting the non-destructive measurement thereof. And, the present inventors studied an approach to define an optimum range of the curing degree of the thermosetting resin based on the correlation between the factor and the residual potential of the electrophotosensitive material.
As a result, the inventors have found that a ratio A2/A1 between absorbances at a first measurement wavelength W1 and a second measurement wavelength W2 (A1 denoting an absorbance at the first measurement wavelength W1, A2 denoting an absorbance at the second measurement wavelength W2) is effective as the above factor. The first measurement wavelength W1 is defined as a predetermined wavelength on a shorter-wave side of a maximum absorption wavelength in a visible absorption spectrum of the intermediate layer, whereas the second measurement wavelength W2 is defined as a predetermined wavelength on a longer-wave side of the maximum absorption wavelength in the spectrum.
It is known that the thermosetting resin, in general, is gradually changed in color as the curing thereof proceeds. Thus the inventors have found that the curing degree of the thermosetting resin can be non-destructively determined by determining the ratio A2/A1 from the change of the color of the resin.
They further examined the correlation between the ratio A2/A1 and the residual potential of the electrophotosensitive material to find that these factors present a correlation distribution as shown in FIG. 1, for example.
Specifically, a electrophotosensitive material belonging to a region representing a greater ratio A2/A1 and a lower curing degree of the thermosetting resin (the right-hand region in the figure) tends to be increased in residual potential in proportion to the decrease of the curing degree, as indicated by the dots in the figure. Such a electrophotosensitive material is more likely to produce fog. Furthermore because of the low curing degree of the thermosetting resin, the electrophotosensitive material belonging to this region has poor thermal, chemical, physical and mechanical stabilities of the intermediate layer.
On the other hand, the ratio A2/A1 is not more than a value corresponding to an intersection X of:
a first approximation line representative of a portion where the increase of the ratio A2/A1 involves little change of the residual potential (a first approximation line) and
a second approximation line representative of a portion where the increase of the ratio A2/A1 involves a proportional increase of the residual potential,
then the residual potential of the electrophotosensitive material varies little, staying at low stable values along the first approximation line. Thus, the electrophotosensitive material is less prone to produce fog. The intermediate layer in this state has such a high curing degree of the thermosetting resin as to achieve good thermal, chemical physical and mechanical stabilities.
Thus, an electrophotosensitive material according to the invention comprises a conductive substrate, an intermediate layer containing a thermosetting resin, and a photosensitive layer, the intermediate layer and the photosensitive layer laminated on the conductive substrate in this order,
wherein the intermediate layer has an absorbance ratio A2/A1 presenting a correlation distribution with respect to a residual potential of the photosensitive material, the absorbance A1 measured at a first measurement wavelength W1 which is a predetermined wavelength on a shorter-wave side of a maximum absorption wavelength in a visible absorption spectrum of the intermediate layer, the absorbance A2 measured at a second measurement wavelength W2 which is a predetermined wavelength on a longer-wave side of the maximum absorption wavelength, and
in the correlation distribution, the ratio A2/A1 is not more than a value corresponding to an intersection of:
a first approximation line representative of a portion where the increase of the ratio A2/A1 involves little change of the residual potential; and
a second approximation line representative of a portion where the increase of the ratio A2/A1 involves a proportional increase of the residual potential.
According to the invention, the thermosetting resin forming the intermediate layer is sufficiently cured, as described above, so that the intermediate layer features not only good thermal, chemical, physical and mechanical stabilities but also a suitable conductivity. Accordingly, the electrophotosensitive material is adapted to achieve a low residual potential of the photosensitive layer for offering fog-free favorable images.
According to the invention, a production method for the above electrophotosensitive material comprises the steps of:
previously deriving the correlation distribution between the ratio A2/A1 of the intermediate layer and the residual potential of the photosensitive material from a plurality of heat treatment conditions for curing the thermosetting resin during the formation of the intermediate layer;
forming the intermediate layer containing the thermosetting resin on the conductive substrate; and
overlaying the photosensitive layer selectively on the formed intermediate layer when the ratio A2/A1 determined from a visible absorption spectrum of the intermediate layer is not more than the value corresponding to the intersection of the first and the second approximation lines in the correlation distribution.
The production method of the invention allows for the determination of the curing degree of the thermosetting resin forming the intermediate layer, before the photosensitive layer is laid thereover. This prevents a defective product having an insufficient curing degree from being committed to the subsequent step. As a result, the electrophotosensitive material having good characteristics can be produced in high yield.
According to the invention, a production method for the above electrophotosensitive material comprises the steps of:
previously deriving the correlation distribution between the ratio A2/A1 of the intermediate layer and the residual potential of the photosensitive material from a plurality of heat treatment conditions for curing the thermosetting resin during the formation of the intermediate layer;
determining from the correlation distribution the heat treatment conditions that provide the ratio A2/A1 of not more than the value corresponding to the intersection of the first and the second approximation lines;
forming the intermediate layer by curing the thermosetting resin under the heat treatment conditions thus determined; and
overlaying the photosensitive layer on the intermediate layer.
The production method of the invention eliminates the possibility of producing the intermediate layer having the low curing degree of the thermosetting resin. As a result, the electrophotosensitive material having good characteristics can be produced in high yield.
According to the invention, a method for inspecting an intermediary before forming the photosensitive layer on the intermediate layer of the electrophotosensitive material according to claim 1 comprising the steps of:
previously deriving the correlation distribution between the ratio A2/A1 of the intermediate layer and the residual potential of the photosensitive material from a plurality of heat treatment conditions for curing the thermosetting resin during the formation of the intermediate layer;
measuring a visible absorption spectrum of the intermediate layer; and
classifying the intermediary into an acceptable one in which the ratio A2/A1 is not more than the value corresponding to the intersection of the first and the second approximation lines, and a defective one in which the ratio A2/A1 is more than the value corresponding to the intersection.
According to the inspection method of the invention, the intermediary of the electrophotosensitive material can be classified into an acceptable one having a high curing degree of the thermosetting resin and a defective one having a low curing degree of the resin by determining the curing degree of the thermosetting resin forming the intermediate layer.